pbdrv/uart_debug_first_port: Log on panic.

It's helpful when you crash to see what happens right before you
crash. For example, if you add a log message "about to dereference
sketchy pointer" and then you get a segfault, you'd love to know
whether that message occurred. This change makes it so that at least
on EV3, we will.

This also adds "pb_log" as an alias for pbdrv_uart_debug_printf.
Usually, platforms make it easy to log debug messages, and we should
be no different. C.f. `ESP_LOGI`, Zephyr's `LOG_ERR`, glog's
`LOG(WARNING)` etc.

Tested by adding the following code to pbdrv_init.

```
    pb_log("test\n");

    lwrb_t* rb = (lwrb_t*)0x1281441490;
    pb_log("%p\n", lwrb_get_free(rb));
```

Before this, the "test" would not appear before the panic.
After, it does. If a panic occurs while some messages are pending
in the uart send state, some bytes may be duplicated on the log.

Author: jaguilar

lib/pbio/drv/uart/uart_debug_first_port.c

@@ -22,6 +22,18 @@ static lwrb_t ring_buffer;
 
 static pbdrv_uart_dev_t *debug_uart = NULL;
 
+int pbdrv_uart_debug_next_char(void) {
+    if (!lwrb_is_ready(&ring_buffer)) {
+        return -1;
+    }
+    uint8_t c;
+    if (lwrb_read(&ring_buffer, &c, 1) == 1) {
+        return c;
+    } else {
+        return -1;
+    }
+}
+
 /**
  * Formats and stores a string in the UART debug ring buffer.
  *

lib/pbio/drv/uart/uart_debug_first_port.h

@@ -19,6 +19,11 @@ bool pbdrv_uart_debug_is_done(void);
 
 void pbdrv_uart_debug_init(void);
 
+// Returns the next character that should be sent to the UART, or -1 if there
+// are not pending log messages. Used only by the panic handler to flush unsent
+// data before reboot.
+int pbdrv_uart_debug_next_char(void);
+
 #else // PBDRV_CONFIG_UART_DEBUG_FIRST_PORT
 
 #define pbdrv_uart_debug_printf(...)
@@ -28,6 +33,13 @@ void pbdrv_uart_debug_init(void);
 
 #define pbdrv_uart_debug_init()
 
+static inline int pbdrv_uart_debug_next_char(void) {
+    return -1;
+}
+
 #endif // PBDRV_CONFIG_UART_DEBUG_FIRST_PORT
 
+// Convenient shorthand for pbdrv_uart_debug_printf.
+#define pbdrv_dbg(...) pbdrv_uart_debug_printf(__VA_ARGS__)
+
 #endif // _INTERNAL_PBDRV_UART_DEBUG_FIRST_PORT_H_

lib/pbio/platform/ev3/platform.c

@@ -81,6 +81,7 @@
 #include "../../drv/pwm/pwm_ev3.h"
 #include "../../drv/reset/reset_ev3.h"
 #include "../../drv/uart/uart_ev3.h"
+#include "../../drv/uart/uart_debug_first_port.h"
 
 enum {
     LED_DEV_0_STATUS,
@@ -443,6 +444,12 @@ void ev3_panic_handler(int except_type, ev3_panic_ctx *except_data) {
     UARTConfigSetExpClk(SOC_UART_1_REGS, SOC_UART_1_MODULE_FREQ, 115200, UART_WORDL_8BITS, UART_OVER_SAMP_RATE_13);
     UARTFIFOEnable(SOC_UART_1_REGS);
 
+    // Drain whatever is still left in the UART debug FIFO.
+    int c;
+    while ((c = pbdrv_uart_debug_next_char()) != -1) {
+        UARTCharPut(SOC_UART_1_REGS, (uint8_t)c);
+    }
+
     panic_puts("********************************************************************************\r\n");
     panic_puts("*                            Pybricks on EV3 Panic                             *\r\n");
     panic_puts("********************************************************************************\r\n");